The present invention relates to pump beam combination techniques, and more particularly to a device which provides polarization beam combining, wavelength division multiplexing, and isolation in the reverse path.
In order to increase pump power input to optical amplifiers such as EDFA, PDFA, and Raman Amplifiers, beams from multiple input lasers are typically combined. FIG. 1 shows an approach typical of the prior art for combining pump input laser beams. In this example, two pump laser beams 105, 110 with wavelength xcex1 and power P but orthogonal polarizations are fed into a first polarization beam combiner (PBC) 120. (Beam 105 is horizontally polarized, i.e. polarized in the plane of the page, while beam 110 is vertically polarized, i.e. polarized perpendicular to the plane of the page.) Similarly, two laser beams 130, 135 with wavelength xcex2 and power P but orthogonal polarizations are fed into a second PBC 145. Through the action of the PBCs 120 and 145, the two beams 105, 110 at xcex1 are combined into a beam having power 2P, as are the beams 130, 135 at xcex2. The combined beams 125 and 150, respectively, then enter a Wavelength Division Multiplexer (WDM) 155, which combines the two beams into a single beam 160 of power 4P (neglecting loss) with wavelengths xcex1 and xcex2. In order to prevent undesirable feedback, for example system noise, along the reverse path to the pump lasers, isolators are used at the output (165) and optionally at the four inputs (115, 140) to the PBCs. Typically, xcex1 and xcex2 will be in the range of about 1400 nm to about 1550 nm but may also be in the range of about 1000 nm to about 1600 nm. The input beams to the PBCs are typically carried through polarization maintaining (PM) optical fibers. The output of the PBCs and WDM are typically carried through single mode (SM) optical fibers.
According to the prior art, therefore, at least four discreet optical components are required to provide a combined pump laser output: two PBCs, one WDM, and at least one isolator.
The invention provides systems and methods for combining optical beams.
In one embodiment, the method comprises providing a first beam angle turner which includes first and second birefringent wedges and a first Faraday rotator disposed between the first and second wedges, providing a second beam angle turner which includes third and fourth birefringent wedges and a second Faraday rotator disposed between the third and fourth wedges, providing a wavelength division multiplexer (WDM) that is disposed between the first and second beam angle turners and that transmits light of a first wavelength and that reflects light of a second wavelength, receiving and combining into a first combined exit beam first and second optical beams of the first wavelength, receiving and combining into a second combined exit beam third and fourth optical beams of the second wavelength, and transmitting the first combined exit beam and reflecting the second combined exit beam. The first and second wedges and the first Faraday rotator are disposed in relation to each other and contoured such that the first and second optical beams incident on the first wedge at a first prescribed separation angle are combined into the first combined exit beam at the first wavelength that exits the second wedge opposite the first Faraday rotator. The third and fourth wedges and the second Faraday rotator are disposed in relation to each other and contoured such that the third and fourth optical beams incident on the third wedge at a second prescribed separation angle are combined into the second combined exit beam that exits the fourth wedge opposite the second Faraday rotator. The first beam angle turner, the second beam angle turner and the WDM are disposed in relation to each other such that that the first combined exit beam that exits the second wedge passes through the WDM and is incident upon the fourth wedge opposite the second Faraday rotator and such that the second combined exit beam that exits the fourth wedge is reflected by the WDM and is incident upon the fourth wedge opposite the second Faraday rotator. The third and fourth wedges, the second Faraday rotator and the WDM are further disposed in relation to each other and contoured such that the first combined exit beam passes through the second beam angle turner and exits the third wedge and follows an exit beam path intersecting the third wedge opposite the second Faraday rotator, the second combined exit beam passes back through the second beam angle turner and exits the third wedge opposite the second Faraday rotator and joins the exit beam path, light of the first wavelength reflected back along the exit beam path passes through the second beam angle turner and passes through the WDM, and light of the second wavelength that is reflected back along the exit beam path passes through the second beam angle turner, is reflected from the WDM and passes again back through the second beam angle turner and exits the third wedge without interfering with either the third or fourth incident beams. The first and second wedges, the first Faraday rotator and the WDM are further disposed in relation to each other and are contoured such that the reflected light of the first wavelength, that is reflected back along the exit beam path through the second beam angle turner and the WDM, passes through the first beam angle turner and exits the first wedge without interfering with either the first or second incident beams. The first combined exit beam comprising the first and second incident beams of the first wavelength and the second combined exit beam comprising the third and fourth incident beams of the second wavelength are combined into a third combined exit beam that follows the exit beam path and that includes the first and second wavelengths. Reflected light reflected back along the exit beam path is isolated from the first, second, third and fourth incident beams.
In another embodiment, the method comprises providing a first beam angle turner which includes first and second birefringent wedges, providing a second beam angle turner which includes third and fourth birefringent wedges and a Faraday rotator disposed between the third and fourth wedges, providing a wavelength division multiplexer (WDM) that is disposed between the first and second beam angle turners and that transmits light of a first wavelength and that reflects light of a second wavelength, receiving and combining into a first combined exit beam first and second optical beams of the first wavelength, receiving and combining into a second combined exit beam third and fourth optical beams of the second wavelength, and transmitting the first combined exit beam and reflecting the second combined exit beam. The first and second wedges are disposed in relation to each other and contoured such that the first and second optical beams incident on the first wedge at a first prescribed separation angle are combined into the first combined exit beam at the first wavelength that exits the second wedge opposite the first wedge rotator. The third and fourth wedges and the Faraday rotator are disposed in relation to each other and contoured such that the third and fourth optical beams incident on the third wedge at a second prescribed separation angle are combined into the second combined exit beam that exits the fourth wedge opposite the Faraday rotator. The first beam angle turner, the second beam angle turner and the WDM are disposed in relation to each other such that that the first combined exit beam that exits the second wedge passes through the WDM and is incident upon the fourth wedge opposite the Faraday rotator and such that the second combined exit beam that exits the fourth wedge is reflected by the WDM and is incident upon the fourth wedge opposite the Faraday rotator. The third and fourth wedges, the Faraday rotator and the WDM are further disposed in relation to each other and contoured such that the first combined exit beam passes through the second beam angle turner and exits the third wedge and follows an exit beam path intersecting the third wedge opposite the Faraday rotator, the second combined exit beam passes back through the second beam angle turner and exits the third wedge opposite the Faraday rotator and joins the exit beam path, light of the first wavelength reflected back along the exit beam path passes through the second beam angle turner and passes through the WDM, and light of the second wavelength that is reflected back along the exit beam path passes through the second beam angle turner, is reflected from the WDM and passes again back through the second beam angle turner and exits the third wedge without interfering with either the third or fourth incident beams. The first and second wedges and the WDM are further disposed in relation to each other and are contoured such that the reflected light of the first wavelength, that is reflected back along the exit beam path through the second beam angle turner and the WDM, passes through the first beam angle turner and exits the first wedge without interfering with either the first or second incident beams. The first combined exit beam comprising the first and second incident beams of the first wavelength and the second combined exit beam comprising the third and fourth incident beams of the second wavelength are combined into a third combined exit beam that follows the exit beam path and that includes the first and second wavelengths. Reflected light reflected back along the exit beam path is isolated from the first, second, third and fourth incident beams.
In another embodiment, the method comprises providing a beam angle turner which includes first and second birefringent wedges, providing a wavelength division multiplexer (WDM) having a first surface facing the first wedge opposite the second wedge that transmits light of a first wavelength and that reflects light of a second wavelength, providing first and second polarization maintaining (PM) fiber terminations facing a second surface of the WDM opposite the first surface, providing a first collimating lens disposed between the second surface of the WDM and the first and second PM fiber terminations, providing third and fourth PM fiber terminations facing the second wedge opposite the first wedge, providing an exit beam optical fiber termination disposed between the third and fourth PM fiber terminations and facing the second wedge opposite the first wedge, providing a second collimating lens disposed between the second wedge and the third and fourth PM fiber terminations and the exit beam optical fiber termination, launching first and second optical beams of the first wavelength, transmitting and combining the first and second optical beams into a first combined exit beam, launching third and fourth optical beams of the second wavelength, and deflecting, reflecting, and combining the third and fourth optical beams into a second combined exit beam. The first and second PM fiber terminations, first collimating lens, and WDM are disposed in relation to each other such that the first and second optical beams of the first wavelength, launched from the first and second PM fiber terminations, respectively, through the first collimating lens and incident on the first wedge at a first separation angle "THgr" are transmitted by the WDM. The WDM and first and second wedges are disposed in relation to each other, and the first and second wedges are contoured such that, the first and second transmitted optical beams of the first wavelength are combined into a first combined exit beam at the first wavelength that exits the second wedge opposite the first wedge. The third and fourth PM fiber terminations, second collimating lens, and first and second wedges are disposed in relation to each other, and the first and second wedges are contoured such that the third and fourth optical beams of the second wavelength, launched from the third and fourth PM fiber terminations, respectively, through the second collimating lens and incident on the second wedge opposite the first wedge at a separation angle 2"THgr" pass through the beam angle turner and are deflected so as to be incident upon the first surface of the WDM at a second separation angle "THgr" and reflected from the first surface. The WDM and first and second wedges are disposed in relation to each other, and the first and second wedges are contoured such that the third and fourth reflected optical beams of the second wavelength are incident upon the first wedge opposite the second wedge and combined into a second combined exit beam at the second wavelength that exits the second wedge opposite the first wedge. The first and second wedges and the WDM are further disposed in relation to each other and contoured such that the first combined exit beam exits the second wedge and follows an exit beam path intersecting the second wedge opposite the first wedge, and the second combined exit beam exits the second wedge opposite the first wedge and joins the exit beam path. The first combined exit beam comprising the first and second incident beams of the first wavelength and the second combined exit beam comprising the third and fourth incident beams of the second wavelength are combined into a third combined exit beam that follows the exit beam path and that includes the first and second wavelengths. The third combined exit beam is received at the exit beam optical fiber termination.